A number of different Echelle polychromator arrangements are known (e.g. U.S. Pat. Nos. 3,658,423; 4,391,523; 4,690,559; and 4,820,048). They have in common the feature that the individual diffraction orders of the Echelle grating must be separated from one another by at least one additional transverse dispersion element within the spectrometer, a two-dimensional spectrum being formed.
The distances between adjacent order lines in the spectrum arise out of the free spectral region of the Echelle diffraction orders and the course of the dispersion of the transverse dispersion element. Prisms are used exclusively for the transverse dispersion in Echelle polychromators which must cover an extended wavelength range, due to their high transmission and their unambiguous dispersion during the diffraction of light. This, however, necessarily results in an uneven separation of the order lines. Corresponding to the course of the dispersion of all prism materials, the order separation increases by a multiple from the long wavelength end of the spectrum at low order numbers to the short wavelength end at high order numbers. The width of the order lines is determined in the Echelle polychromator by the width of the transverse slit, which as entrance slit component limits the light bundle in the direction of the prism dispersion.
In all known Echelle polychromator arrangements which use a prism for the order separation, heretofore the width of the transverse slit has always been selected so that the longest wavelength order lines in the Echelle spectrum are adjacent to one another with only a minimum space between them to allow for distortion aberration.
Since the width of the transverse slit is the same for all wavelengths, the size of the free areas without informational content between the orders increases in the spectrum as the order number increases. Due to a wider transverse slit, these free areas could be used to increase the luminosity. However, the overlapping of the orders in the long wavelength region of the spectrum, which is associated with this, can generally not be tolerated.
The luminosity of an Echelle polychromator is thus limited by the small width of the transverse slit and is constant over the whole of the spectral range. The diffraction efficiency of the Echelle grating, the reflection coefficients of the mirror optics, and the transmission of the prism material, however, decrease in the direction of the shorter wavelengths.
Moreover, the signal-to-noise ratio in spectrophotometric measurements in the ultraviolet region of the spectrum, particularly in the case of sources with maximum radiation density in the visual or NIR range, is reduced further by an increased level of scattered light and is insufficient in many applications.